This invention relates to a process for preparing unsymmetric and/or symmetric dialkyl carbonates and diols. More specifically the present invention relates to a process for preparing dialkyl carbonates and diols from the reaction product of cyclic carbonates, hydroxy alkyl carbonates and alcohols.
Dialkyl carbonates are important intermediates for the synthesis of fine chemicals, pharmaceuticals and plastics and are useful as synthetic lubricants, solvents, plasticizers and monomers for organic glass and various polymers, including polycarbonate, a polymer known for its wide range of uses based upon its characteristics of transparency, shock resistance and processability.
One method for the production of polycarbonate resin employs phosgene and bisphenol-A as starting materials. However, this method has numerous drawbacks, including the production of corrosive by-products and safety concerns attributable to the use of the highly toxic phosgene. As such, polycarbonate manufacturers have developed non-phosgene methods for polycarbonate production, which use diphenyl carbonate and bisphenol-A as starting materials. Diphenyl carbonate can be prepared from phenol and dimethyl carbonate.
Dimethyl carbonate has a low toxicity and can also be used to replace toxic intermediates, such as phosgene and dimethyl sulphate, in many reactions, such as the preparation of urethanes and isocyanates, the quaternization of amines and the methylation of phenol or naphthols. Moreover, it is not corrosive and it will not produce environmentally damaging by-products. Dimethyl carbonate is also a valuable commercial product finding utility as an organic solvent, an additive for fuels, and in the production of other alkyl and aryl carbonates.
Dimethyl carbonate, as well as other dialkyl carbonates, have traditionally been produced by reacting alcohols with phosgene. These methods have the same problems as methods that use phosgene and bisphenol-A, i.e., the problems of handling phosgene and disposing of phosgene waste materials. Thus, there is a need for commercially viable non-phosgene methods for the production of dimethyl carbonate, as well as other dialkyl carbonates.
Non-phosgene methods that have been proposed for producing dialkyl carbonates include the transesterification reaction of alcohols and cyclic carbonates. Most of the proposed methods relate to the use of various catalysts for that reaction. Examples of such proposed catalysts include alkali metals or basic compounds containing alkali metals; tertiary aliphatic amines; thallium compounds; tin alkoxides; alkoxides of zinc, aluminum and titanium; a mixture of a Lewis acid and a nitrogen-containing organic base; phosphine compounds; quaternary phosphonium salts; cyclic amidines; compounds of zirconium, titanium and tin; a quaternary ammonium group-containing strongly basic anion-exchange solid material; a solid catalyst selected from the group consisting of a tertiary amine- or quaternary ammonium group-containing ion-exchange resin, a strongly acidic or a weakly acidic ion-exchange resin, a mixture of an alkali metal with silica, a silicate of an alkaline earth metal and an ammonium ion-exchanged zeolite; and a homogeneous catalyst selected from the group consisting of tertiary phosphine, tertiary arsine, tertiary stibine, a divalent sulfur compound and a selenium compound.
The catalytic transesterification of a cyclic carbonate with an alcohol generally involves two equilibrium steps which typically generates a hydroxyalkyl carbonate as the reaction intermediate. For example, in the transesterification of ethylene carbonate (EC) with methanol (MeOH), the intermediate which is formed is 2-hydroxyethyl methyl carbonate (HEMC). This two equilibrium step reaction may be represented by the following: 
These reaction steps for converting the cyclic carbonate and alcohol to the dialkyl carbonate generally occur as two sequential steps. Addition of the first molecule of alcohol to the cyclic carbonate results in the production of the intermediate hydroxy alkyl carbonate. Addition of the second molecule of alcohol to the intermediate results in the production of the dialkyl carbonate and diol. The intermediate hydroxy alkyl carbonate generally builds to a maximum concentration faster than the equilibrium dialkyl carbonate concentration is reached. As a result of equilibrium constraints on the reactions, a maximum concentration (i.e., the equilibrium concentration) will be reached for the desired products. Thus, there is a limit to the yield for producing dialkyl carbonates and diols from cyclic carbonates and aliphatic monohydric alcohols for a given catalyst and reaction conditions.
Unsymmetric dialkyl carbonates, such as ethyl methyl carbonate (EMC), are useful as solvents for electrolytic solutions for lithium rechargeable batteries, solvents for resins and coating compositions, alkylating agents, or starting materials for carbamate synthesis.
Ethyl methyl carbonate, as well as other unsymmetric dialkyl carbonates, have traditionally been produced by esterification of alkyl chloroformate with alcohol under base (pyridine or amine) catalysis. Such methods have similar problems to the methods discussed above that use phosgene and bisphenol-A, i.e., highly reactive and highly toxic starting materials.
Other methods have been disclosed for the synthesis of unsymmetric dialkyl carbonates, which avoid the use of such highly toxic starting materials. One method involves an ester exchange reaction of a symmetric dialkyl carbonate with an alcohol having a different alkyl group under base catalysis. However, such a reaction typically results in a product, which includes a mixture of three dialkyl carbonates and two alcohols. For example, when a 1:1 molar ratio of DMC and EtOH is used as the starting materials, the product mixture will typically contain about a 45:45:10 molar ratio of DMC:EMC:DEC (diethyl carbonate) and a relative ratio of MeOH to EtOH of about 2:1. The mixture of these three dialkyl carbonates can result in difficult or costly purification steps to isolate the unsymmetric dialkyl carbonate, e.g., EMC.
Other methods which have been proposed include the disproportionation of two symmetrical dialkyl carbonates using a basic catalyst, e.g., an alkali metal alcoholate. However, such methods typically result in a product mixture of three dialkyl carbonates, including one unsymmetrical dialkyl carbonate. Again, this can result in difficult or costly purification steps to isolate the unsymmetric dialkyl carbonate from the three component mixture.
Thus, there is a need for a process for the production of symmetric and/or unsymmetric dialkyl carbonates and diols from starting materials which include cyclic carbonates and alcohols which does not have the above-mentioned disadvantages.
According to the present invention, it has now been found that unsymmetric and/or symmetric dialkyl carbonates and diols, and more specifically dimethyl carbonate and ethylene glycol, can be prepared with higher yields, from a feed containing a cyclic carbonate, a hydroxy alkyl carbonate and an aliphatic monohydric alcohol, compared to a feed containing only a cyclic carbonate and an aliphatic monohydric alcohol. In another aspect, it has been found that unsymmetric dialkyl carbonates can be produced, along with symmetric dialkyl carbonates, by selection of the hydroxy alkyl carbonate(s) present in the feed such that the hydroxy alkyl carbonate has an alkyl group which is different from the alkyl group in the aliphatic monohydric alcohol reactant.
The process of the present invention involves reacting a cyclic carbonate and a hydroxy alkyl carbonate with an aliphatic monohydric alcohol in the presence of a transesterification catalyst in a transesterification reaction zone to provide a dialkyl carbonate and a diol.
Preferably, the cyclic carbonate of the present invention is of the formula: 
the hydroxy alkyl carbonate is of the formula: 
the aliphatic monohydric alcohol is of the formula:
R4xe2x80x94OHxe2x80x83xe2x80x83(IV)
wherein R1 and R2 independently of one another denote a divalent group represented by the formula xe2x80x94(CH2)mxe2x80x94, wherein m is an integer from 1 to 3, which is unsubstituted or substituted with at least one substituent selected from the group consisting of a C1-C10 alkyl group and a C6-C10 aryl group, wherein R1 and R2 can share the same substituent; and R3 and R4 independently of one another denote a monovalent aliphatic C1-C12 hydrocarbon group which is unsubstituted or substituted with at least one substituent selected from the group consisting of a C1-C10 alkyl group, a C2-C10 vinyl group or a C6-C10 aryl group.
In a preferred embodiment, the cyclic carbonate is ethylene carbonate, the hydroxy alkyl carbonate is 2-hydroxyethyl methyl carbonate, the aliphatic monohydric alcohol is methanol, the dialkyl carbonate is dimethyl carbonate and the diol is ethylene glycol.
The present invention provides the advantage of producing the desired dialkyl carbonates and diols in higher yield than that of a process which reacts only the cyclic carbonate and aliphatic monohydric alcohol. Also, the use of hydroxy alkyl carbonates having different alkyl groups than the alcohol provides unique mechanisms for producing unsymmetric dialkyl carbonates. The source of the hydroxy alkyl carbonate can be from a recycle stream of the present process or from any other source, including the product from other processes.
An integrated process for the production of a dialkyl carbonate and a diol from an alkylene oxide, carbon dioxide and an aliphatic monohydric alcohol comprising: (a) reacting an alkylene oxide with carbon dioxide in the presence of a carbonation catalyst at a temperature in the range of about 50xc2x0 C. to 250xc2x0 C. and at a pressure of at least about 1379 kPa (200 psi) to provide a crude cyclic carbonate stream comprising a cyclic carbonate and carbonation catalyst; and (b) reacting the cyclic carbonate and a hydroxy alkyl carbonate with the aliphatic monohydric alcohol in the presence of a transesterification catalyst, thereby producing a crude product stream comprising the dialkyl carbonate, the diol and the hydroxy alkyl carbonate; and (c) separating the hydroxy alkyl carbonate from the crude product stream and recycling the hydroxy alkyl carbonate to step (b).
Additional objects, advantages and novel features of the invention will be set forth in part in the description and examples which follow, and in part will become apparent to those skilled in the art upon examination of the following, or by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.